Re: [RFC PATCH 48/86] rcu: handle quiescent states for PREEMPT_RCU=n

[Steven Rostedt]

On Tue, 21 Nov 2023 13:14:16 -0800
"Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote:

> On Tue, Nov 21, 2023 at 09:30:49PM +0100, Peter Zijlstra wrote:
> > On Tue, Nov 21, 2023 at 11:25:18AM -0800, Paul E. McKenney wrote:  
> > > #define preempt_enable() \
> > > do { \
> > > 	barrier(); \
> > > 	if (!IS_ENABLED(CONFIG_PREEMPT_RCU) && raw_cpu_read(rcu_data.rcu_urgent_qs) && \
> > > 	    (preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK | HARDIRQ_MASK | NMI_MASK) == PREEMPT_OFFSET) &&
> > > 	    !irqs_disabled()) \

Could we make the above an else case of the below if ?

> > > 		rcu_all_qs(); \
> > > 	if (unlikely(preempt_count_dec_and_test())) { \
> > > 		__preempt_schedule(); \
> > > 	} \
> > > } while (0)  
> > 
> > Aaaaahhh, please no. We spend so much time reducing preempt_enable() to
> > the minimal thing it is today, this will make it blow up into something
> > giant again.  

Note, the above is only true with "CONFIG_PREEMPT_RCU is not set", which
keeps the preempt_count() for preemptable kernels with PREEMPT_RCU still minimal.

-- Steve